1. Field of Use
The invention will be used in the construction industry for preparing in-place soil-cement or for mixing haul-in soil-cement commonly used for the base layer under Portland cement concrete or asphalt concrete roads, parking lots, runways, taxi ways, aporns and the like; and in the construction of land fill containment bases and dikes. Soil-cement in this definition includes in-place native soil, natural aggregate such as gravel, processed aggregate such as washed sand, and waste or reclaimed material such as reground asphalt road bed. The process of incorporating cement into road base material is commonly known as Cement Treated Base or "CTB" operations.
2. Prior Art
Construction methods for preparing a base for a road bed or the like before CTB operations commence, vary considerably with intended use, strength requirements, and whether new construction or rehabilitation of an old road. CTB operations start when the base material (native soil, subsoil, plus any haul-in aggregates and/or reclaimed material) are placed to specified depth and grade. A current method employed by road builders commonly includes:
a. Spreading dry cement in a uniform layer on top of the base material.
b. Mixing the dry cement into the base material on the top part of the base material.
c. Adding extra water as needed to bring the total moisture of the CTB layer to optimum percentage needed for maximum density compaction.
d. Mixing the total CTB layer to provide uniform cement and moisture throughout.
e. Shaping, grading and compaction to approximate grade.
f. Finishing to final base grade and plan.
g. Keeping the surface of the CTB damp until covered by a cure coat of asphalt or other material.
After a curing period of one to fourteen days, the CTB surface is then covered with the final layer of concrete or asphalt concrete.
Portland cement owes most of its strength producing properties to chemical hydration, a process where liquid water combines with the cement particles to form crystalline water. Thus, most efficient use of the cement (of highest strength) in respect to the amount of cement used is attained when mixing, shaping, grading and compaction are complete before a significant amount of chemical hydration of the cement occurs. Gypsum is interground with cement clinker to provide a basic hydration delay of 1/2 to 2 hours for Portland cement, depending on the ambient temperature. However, to fully activate the retardation provided by the gypsum, a continuous water phase must be present. Cement clinker, without the retardation influence of gypsum, hydrates in less than 15 minutes. Since moisture is in the base material and the temperature of the base material is influenced by weather conditions, large variations in the strength produced can occur when using dry cement addition. The lack of sufficient time to complete compaction and finishing of the CTB and the absence of any convenient means to delay the hydration time are inherent deficiencies of using dry cement in CTB operations.
Cement dust generated by spreading and mixing dry cement into base material is always hazardous to some extent to the area surrounding a CTB construction site. The fine, highly alkaline silicate particles can damage delicate vegetation, painted surfaces, and is hazardous to skin, lungs, and eyes of humans and animals. Cement dust is especially objectionable in heavily populated areas and areas of high vehicle traffic.
Using cement slurry prepared by a method which eliminates cement dust at the construction site is a direct solution to aforementioned deficiencies prevalent when using dry cement spreading. Such a method was shown in U.S. Pat. No. 5,064,292 issued to John S. Sutton, Nov. 12, 1991. The method shown therein produced a slurry ideally suited for CTB construction. However, that method, required off-site mixing to eliminate the cement dust, required batch weighing equipment, transportation time from batch plant to construction site, and the mixing trucks used were excessively heavy with poor maneuverability for spreading the slurry. The mix rate per unit was too slow for many large scale CTB operations requiring several units to keep pace.
Further, the additional retarder required for locations more than 60 minutes from the batch plant can easily result in slow strength development which can delay placement of the surface course of concrete or asphalt material. Still further slurry mixing rate and changes were at the mercy of other demands on the batch plant.
U.S. Pat. No. 5,064,292, is incorporated herein by reference.